The problem of more effective utilization of coal is now being attacked on an emergency basis as the result of the approaching end to the availability of oil. A number of plans have been devised for circumventing the expense and danger of mining. Open-pit mining and strip mining have only limited application since tremendous quantities of coal are positioned deep in the earth. To obtain the energy locked into coal in coal seams, the ancient process of deep-pit mining has been the only available means.
Recently, processes for chemical comminution of coal, both above ground and below ground have been disclosed in U.S. Pat. Nos. 3,815,826, 3,850,477, 3,870,237 and 3,918,761. According to the processes taught in these patents the interlayer forces at natural interfaces present in the coal is weakened by contact with a number of reagents such as gaseous anhydrous ammonia, liquid anhydrous ammonia, aqueous ammonia, organic solvents with molecular weights lower than 100, and alkali. As a result of such weakening of interlayer forces the coal fractures either spontaneously or with the expenditure of substantially less energy than is usually necessary.
These patents teach the treatment of coal in underground coal seams for the purpose of removing the coal from such seams to the surface. Once the coal is brought to the surface, shipment of the coal to the area of use is then envisioned.
Since bringing the coal to the surface and shipment of the coal to the user are both expensive, attempts have been made to extract the energy of the coal while it is still underground. Foremost among such attempts has been the development of underground coal gasification to produce a combustible gas which can then be transported by pipeline. While considerable progress in the development of this process has been made, the conversion efficiency remains disappointing. This difficulty stems from a number of sources, outstanding among which is the low permeability of coal to the flow of gas therethrough. As is evident, combustion cannot be carried out efficiently unless an oxygen-containing gas can be passed through the coal seam. To cope with this problem, it has been the practice to introduce explosives into the coal seam through bore holes for the purpose of fracturing the coal. Pneumatic and hydraulic fracturing are also sometimes utilized. Unfortunately, it frequently happens that fracturing takes place beyond the boundaries of the coal seam so that water can leak into the seam from the over-burden and gas can be lost from the seam. Moreover, fracturing may not be evenly distributed throughout the seam leading to under-utilization of the coal during gasification. Also, it is necessary to shut down the operation when it is desired to start combustion in a new portion of the seam. This makes for frequent cessation of operation and increase in cost. Also, the area which can be effectively broken up by prior means is limited so that, in general, the maximum size of a panel of coal which can be burned in a single step after such preparation is about 100 ft. .times. 100 ft.
Lignite coal is the best type for use in the gasification process, but even with this type of coal, conversion efficiency is about 60% at best. As the rank of the coal ascends the conversion efficiency drops off and, finally, bituminous coal has been found to be extremely unsuitable for the gasification process as hitherto practiced, due to the fact that it swells and becomes impermeable to gas flow therethrough. Since a large portion of the available coal is bituminous, the non-reactivity of this type of coal in the coal gasification process constitutes a serious limitation on the applicability of the process.
As is evident, then, it would be highly desirable to be able to increase the efficiency of the conversion process with the most suitable coals as well as to be able to render bituminous coal and coals of higher rank suitable for use in coal gasification.